27 July 2021
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Original Citation:
Electrochemical and Photochemical Reduction of CO2 Catalyzed by Re(I) Complexes Carrying Local Proton Sources
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DOI:10.1021/acs.organomet.8b00588 Terms of use:
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Electrochemical and Photochemical Reduction of CO
2
Catalyzed by Re(I) Complexes Carrying Local Proton
Sources
Laura Rotundo,a Claudio Garino,a Emanuele Priola,a Daniele Sassone,a Heng Rao,b Bing Ma,b Marc Robert,b Jan Fiedler,c Roberto Gobetto*a and Carlo Nervi*a
a University of Torino, Department of Chemistry, via P. Giuria 7, 10125 Torino, Italy.
b Université Paris Diderot, Sorbonne Paris Cité, Laboratoire d’Electrochimie Moléculaire, UMR 7591 CNRS,
15 rue Jean-Antoine de Baïf, F-75205 Paris Cedex 13, France
c The Czech Academy of Sciences, J. Heyrovský Institute of Physical Chemistry, Dolejškova 3, 18223 Prague,
Czech Republic
KEYWORDS. Electrochemistry, Photochemistry, CO2 Reduction, Manganese, Rhenium.
ABSTRACT.
The novel rhenium complexes fac-Re(pdbpy)(CO)3Cl (pdbpy = 4-phenyl-6-(phenyl-2,6-diol)-2,2’-bipyridine),
1, and fac-Re(ptbpy)(CO)3Cl (ptbpy = 4-phenyl-6-(phenyl-3,4,5-triol)-2,2’-bipyridine), 2, have been
synthesized and the single crystal X-Ray diffraction (SC-XRD) structure of 1 solved. The electrochemical behaviors of the complexes in acetonitrile under Ar and their catalytic performances for CO2 reduction with
added water and MeOH are discussed. A detailed IR spectroelectrochemical study under Ar and CO2
atmospheres coupled with DFT calculations allows the identification of reduced species and the interpretation of the reduction mechanisms. Comparison between the rhenium complexes and the corresponding Mn derivatives Mn(pdbpy)(CO)3Br, 3, and Mn(ptbpy)(CO)3Br, 4, has been also considered. Finally,
nm) in acetonitrile as solvent. Remarkably, 1 and 3 catalysts were active towards CO2: producing formate with
good selectivity and turnover number (TON). For example, 3 gives 62% selectivity for HCOO– and a TON of 80 and the Re compound 1 gives 74% selectivity for HCOO- and a TON of 86.
Introduction.
The overall world energy demand is constantly increasing, as well as the anthropogenic emissions of CO2, its
concentration in the atmosphere, and the associated environmental ravages. Undoubtedly the catalytic conversion of CO2 is an attractive option that may allow producing chemicals and fuels upon energy storage,
and that would help mitigating greenhouse effects in the mid-term (if the gas conversion could be achieved at the Gt scale, which remains currently unachieved).1, 2 Worldwide energy data show that solar energy production by photovoltaics underwent a remarkable growth during last years, thus favouring the ongoing process of the long and complicated transition from fossil fuels to renewable energy.3 However, the approach to produce sunlight energy in form of electricity still lacks from the point of view of high-density, long-term stability and suffers its intermittent nature and yet unsolved storage issue. The sunlight energy storage/conversion problem has not a unique and definitive solution, so that efforts in broadening the type of methodologies of the approaches should be pursued. Electrochemical processes driven by solar photovoltaic or direct photochemical reductions of CO2 are recently undergoing an impressive upsurge of interests, in the hope to establish an
efficient and reliable artificial carbon-cycle as energy vector for solar energy conversion.4-9
The choice of an efficient and selective catalyst for electrochemical reduction of CO2 is mandatory.2 While
heterogeneous electrocatalysis is a promising approach in terms of durability, stability and improved TON efficiencies,10-12 a rational approach based on the design of efficient homogeneous catalyst still has several benefits,13, 14 including the possibility to covalently attach the molecular catalysts on the electrode surface.15 While this paper was under evaluation, Mn and Re catalysts for CO2 electrochemical reduction were chemically
bonded on different electrode surfaces,16, 17 and new Cu and Mn photocatalysts employed,18 displaying very interesting conversions and TONs. In the search of efficient homogeneous catalysts for CO2 reduction many
groups explored organometallic complexes,19-23 including alternatives to Re bpy-type complexes.24-27 Deronzier and co-workers replaced Re with Mn,28 and later we extended the concept of local proton source (first applied to iron porphyrins29) to Mn.30-32 Ready (entropically) available local protons greatly enhance the catalytic
activity of CO2 reduction. The previously synthesized complex 3, carrying the pdbpy ligand (Scheme 1),
generates formate via metal hydride species,30, 31 in addition to the classical CO production, even in anhydrous acetonitrile. A significant improvement of the catalytic activities has been observed introducing the charge through space effect on iron porphyrins, both in electro-33 and photo-catalysis.34 However, the local proton source approach, albeit not fully understood, still has some advantages (sometimes unexpected as in the case of water oxidation35), like the possibility to change and tune the selectivity. In this paper, whose aim is to highlight differences between Mn and Re complexes carrying the same ligand, we explore the photo- and electro-catalytic activities of novel Re complexes (1 and 2) with pdbpy and ptbpy ligands (Scheme 1) and report the mechanism for the CO2 catalytic reduction. We also report for the first time the photocatalytic activities of Mn
complexes 3 and 4. Comparison with the better-known corresponding Mn complexes helps in the understanding of the catalytic activities of such type of catalysts for homogeneous CO2 reduction.
Scheme 1. Complexes investigated. (M=Re, X=Cl): 1, Re(pdbpy)(CO)3Cl; 2, Re(ptbpy)(CO)3Cl; (M=Mn,
X=Br); 3, Mn(pdbpy)(CO)3Br; 4: Mn(ptbpy)(CO)3Br
Results and Discussion Synthesis and structure
The novel rhenium complexes fac-Re(pdbpy)(CO)3Cl (pdbpy =
4-phenyl-6-(phenyl-2,6-diol)-2,2'-bipyridine), 1 and fac-Re(ptbpy)(CO)3Cl (ptbpy=4-phenyl-6-(phenyl-3,4,5-triol)-2,2-bipyridine), 2, have been
synthesized according to the procedure reported in the experimental section. Single-crystal X-Ray diffraction data of 1 has been obtained from yellow prismatic crystal, grown by slow evaporation under dark conditions
from 1 sp positi ellips to the prese O5-H of we disord a) Figur struct Cycli The comm electr metal type)( vs. A a toluene/d pace group. ion of the soids show a e α,α’-diim nce of intra H5…Cl1 = 3 eak hydrog dered toluen re 1. Mole tural channe ic Voltamm e electroche mon Re(bpy rochemicall l.37 The rad (CO)3Cl rad Ag/AgCl) an dichloromet . The metal ligand is a a weak barr mine plane (d amolecular 3.102(10) Å gens bonds ne molecule ecular struc els (b). metry emical beha y)(CO)3Cl s y reversibl dical anion dical anions nd forms, a hane 1:1 so llic center h almost copl rier to rotati dihedral an hydrogen b Å, see Table and is sta es that show cture of 1 aviour of 1 system.24, 36 e, localized is usually s s may releas fter Cl– dis olution. Com has a slightl lanar with ion. The dip ngle of 72.5 bond interac e S1). The o abilized by w C-H….π i from X-Ra in anhydro The 0/1– re d on the or stable in CV se Cl–.36 Th ssociation, t mpound 1 ( ly distorted the aromat phenolic rin 5°). This pa ction with th overall pack the presen interactions b) ay single cr ous MeCN eduction of rganic ligan V time scal he 1–/2– redu the interme
(Fig. 1a) cry d octahedral
tic bpy sys ng in the 6’ art of the str he chlorine king of the m nce of chan (Figs. 1b a rystal diffra under Ar a f 1 (E° = –1 nd with a le, but in c uction of 1 i diate penta ystallizes in l geometry. stem (torsio position is ructure is m bonded to molecular co nnels runni nd S1). action (a) atmosphere 1.30 V vs. A non-negligi ertain cond is chemicall -coordinate n a triclinic The pheny onal angle instead nea more ordere the central omplex sho ing along a and represe (Fig. 2) res Ag/AgCl) i ible contrib ditions and ly irreversib ed anion, [R centrosymm yl group in of 32°), bu arly perpend ed because metallic at ows a multip an axis fill entation of sembles tha is chemicall bution of th solvents Re ble (Ep = –1 Re(pdbpy)(C metric the 4’ ut the dicular of the tom (d plicity led by f axial at of a ly and he Re e(bpy-1.59 V CO)3]–
acting comm reduc curren Figur scan r The r secon DFT [Re(b the w and o mono to an g as a precu monly assig ction and lo nt peak is p re 2. Cyclic rate 200 mV reversibility nd electron calculations bpy)(CO)3C weakening o only 1.7 kJ/ oanion [Re(b optimized ursor for CO gned to a li ocated betw proportional c voltammo V/s. The bla y of the 0/1– transfer. Th s on the rad Cl]– and [Re of the Re-Cl /mol for 1– bpy)(CO)3C geometry in O2 binding a igand-centre een –0.5 an to the scan grams of 1 ack line is th – reduction o his is nicely dical anion 1 (bpy)(CO)3 l bond after – and 12–, r Cl]– is 40.1 n which the and subsequ ed reduction nd –0.8 V i n rate (for ex mM solutio he electroly of 1 (see als y highlighte 1– and the d 3Cl]2– are 2. r 2e reductio respectively kJ/mol, wh e Re-Cl bon uent reductio n. The bro is due to ad xample see on of 1 in M yte saturated so Fig.S3) s ed by relaxe dianion 12–. .779, 2.913 on. The dis y (Fig. S4). hile it is barr nd, 4.609 Å,
on. The thir ad reoxidat dsorbed spe Fig. S2 at 1 MeCN / 0.1 d with CO2. suggests tha ed geometry Re–Cl bond , 2.776 and ssociation o The energ rierless in th , is already rd process a tion process cies on the 00V/s).38, 39 M Bu4NPF t the release y scan of th ds in the op 4.609 Å, re f Cl– in Me getic barrier he dianion [ broken). Th at more nega s observed electrode s 9 F6 at glassy e of Cl– onl he Re–Cl bo ptimized stru espectively eCN as solv r for the di [Re(bpy)(C hus, the cal
ative potent after the s surface, sin carbon elec y occurs aft ond perform uctures of 1 , showing c vent require issociation CO)3Cl]2– (le lculations su tials is second nce the ctrode, fter the med by 1–, 12–, clearly s 32.0 of the eading uggest
that p in the very s The (incre other reduc The acids 1 with Figur the ba pdbpy in 1– e same cond small barrie e CV of c eased curren bipyridine ction peak.20 e catalytic p : H2O and M h CO2 on th re 3. Cyclic ackground u slightly fac ditions. Equ er. complex 1 nt) already a -based rhen 0, 24 performance MeOH. CVs he first and p c voltammo under CO2. cilitates the ually impor in anhydro at the first r nium cataly e of 1 has b s in Figure 3 particularly grams of 1 release of tant is the f ous MeCN reduction pe ysts, where
een also inv 3 show that y on the seco
mM solutio
Cl– from th fact that rel
under CO eak (Ep = –1 the catalyt vestigated w t the additio ond peak. ons of 1 in M he complex, lease occurs O2 atmosphe 1.30V, Fig. tic activity
with the add on of 5% of MeCN / 0.1 , when com s from the d ere exhibits 3). This is a is observed dition of two the two aci
1 M Bu4NPF mpared to [R doubly redu s a small a notable di d in CV on o different e ids enhance F6 at 200 m Re(bpy)(CO uced specie catalytic ac fference fro nly at the s external Brø s the reactiv mV/s; black O)3Cl]– s with ctivity om the second ønsted vity of line is
Cyclic voltammetry of 2 Re(ptbpy)(CO)3Cl, under Ar in anhydrous MeCN (Fig.4) exhibits three consecutive
reduction processes. The first peak (at Ep= –1.38 V vs. Ag/AgCl) corresponds to the formation of the radical
anion [Re(ptbpy)(CO)3Cl]–, and differently from 1, shows a certain degree of chemical irreversibility (higher
scan rates improves the reversibility). We were unable to completely remove water from compound 2 (a little amount of water has been always found during NMR characterization), resulting in a less reversible 0/1– reduction, as well as altering the CV under CO2. Furthermore, reproducibility of the CVs was an issue on GCE
(glassy carbon electrode) because of the adsorption of reduction products or side-products causing an inactivation of the electrode surface. Similarly to 1, two other reduction peaks are seen at –1.77 and –2.13 V vs. Ag/AgCl.
The electrochemical behavior of 2 in anhydrous MeCN under CO2 atmosphere (Fig. 4) shows a net catalysis
already at the first reduction peak. As mentioned above, this is due to small amounts of water retained in the more hydrophilic solid sample of 2. Significant improvement is observed when MeOH or large amount (5%) of water are added. The CV peak currents decrease after consecutive CVs, but GCE cleaning restores the original CV with high catalytic current.
Fig (blue) Spect of spe Infr monit growi for th Figur reduc cm–1) chara (the r gure 4. CVs ), under CO troelectroc ectral band rared spectr tor the spec ing and dec he assignme re 5 shows ction peak. A ) was obser acterized by reversed pot s of a 0.5 mM O2 and 5% H hemistry in ds roelectroche ctral change creasing of ent of the pr spectral ch As a main rved along y IR absorpt tential scan M MeCN so H2O (navy) n optically emistry (IR es of the CO the corresp roducts and hanges durin feature a pr with the s tions at 201 does not le olution of 2 and under C transparen R-SEC) of c O stretching ponding ban intermediat ng the pote rogressive l imultaneou 10, 1901 and ead to the or 2 at 200 mV CO2 and 5% nt thin-laye complex 1 g bands (νC nds. DFT ca te species o ential scan loss of the us growth o d 1879 cm– riginal spect V/s under Ar % MeOH (vi er electrode in acetonit CO). For all alculations observed dur up of –1.4 νCO stretche of a new sp –1. The redu trum), howe r (red and gr iolet) at a gl e (OTTLE) trile under IR-SEC fig have been u ring IR-SEC V vs. Ag/A es of compl pecies (1d, uction in the ever this is reen curve) lassy carbon ) cell and D Ar was per gures the arr used as com C bulk elect /AgCl, i.e ju lex 1 (2023 in Table 1 e OTTLE c not in contr , under CO2 n electrode DFT calcula rformed so rows indica mplementar trolysis und ust after th 3, 1918 and 1 and Schem cell is irreve radiction wi 2 ations as to ate the ry tool der Ar. he first d 1901 me 2) ersible ith the
CV reversibility, because of the quite different time scales of CV and IR-SEC.31 The longer time scale in IR-SEC experiments allows ligand exchange between Cl– and a solvent molecule and the subsequent rearrangement to 1d (Scheme 2). The decrease in intensity of the OH stretches at 3318 cm–1 (inset in Fig. 5) suggests that 1d is a species originated from a reductive single deprotonation of the starting complex 1, i.e. 1d is formulated as ([Re(CO)3(pdbpy−H+)]). A similar reductive deprotonation has already been observed for the
analogous Mn complex.31 2050 2000 1950 1900 1850 1800 0.00 0.05 0.10 0.15 0.20 3600 3400 3200 0.00 0.01 0.02 Ab so rban c e / cm-1 3358 Absorbanc e / cm-1 2023 2010 2017 2002 1918 1901 1879
Figure 5. IR-Spectroelectrochemical reduction of a 2.9 mM solution of 1 in MeCN / 0.1 M Bu4NPF6 under Ar
at the first reduction wave (final potential –1.4 V vs. Ag/AgCl). The inset shows the decrease of νOH.
The transition from 1 to 1d (bands at 2010, 1901 and 1879 cm–1, see Fig. 5 and Scheme 2) proceeds via the formation of other intermediate species, as revealed by the presence of νCO at 2017 and 2002 cm–1, which
finally converge into the peak at 2010 cm–1. Two possible intermediates could be involved in the transformation.20, 40 The band at 2002 cm–1 is attributed the radical anion 1a (1998 cm–1 observed for the analogous complex with unsubstituted bipyridine),40 and the band at 2017 cm–1 could be due to the corresponding radical 1b obtained by substitution of Cl– by MeCN (2011 cm–1 observed for bpy analogue).40 In conclusion, 1d is the predominant species at the end of the first reduction in IR-SEC time scale experiments.
IR-SEC experiments with Et4NCl (tetraethylammonium chloride) have been performed with the aim of
absorptions, and the starting νCO at 2020, 1916 and 1894 cm–1 of 1 are transformed, after reduction at the first
wave, into νCO at 2006, 1993, 1894 and 1874 cm–1. These four absorption bands can be seen as a sum of IR
spectra of the species 1d and additional product(s). The band 1993 can be ascribed to the anion [Re(CO)3(pdbpy)MeCN]– (1c) which is formed in limited amount due to close reduction potentials of 1 and 1b.
Formation of analogous [Re(CO)3(bpy)MeCN]– with bands 1986, 1868, 1852 cm–1 was observed during
reduction of [Re(CO)3(bpy)Cl] in similar conditions.40 IR bands at 2002, 1891, 1871(sh) cm–1 appearing in the
early stage of the electrolysis can be assigned to the radical anion 1a (DFT computed at 1999, 1892 and 1875 cm–1) formed as intermediate. Also, a band at 2017 cm–1 attributable to the neutral radical 1b (DFT computed at 2017, 1917, 1899 cm–1) appears temporarily. The corresponding experimental values for [Re(bpy)(CO)3Cl]•–
and [Re(bpy)(CO)3(MeCN)]• in MeCN were reported to be 1998, 1885, 1866 cm–1 and 2011, 1895(br) cm–1,
respectively,40 in excellent agreement with calculations. The presence of Et4NCl partially suppresses the Cl–
dissociation, but eventually in a longer time scale the 1e reduction still ends in the conversion to 1d.
2050 2000 1950 1900 1850 1800 0.00 0.05 0.10 0.15 0.20 Absorbance / cm-1 2020 2006 2002 1993 1916 1894 1874 1894
Figure 6. IR spectroelectrochemistry of 1 in MeCN/0.1 M Et4NCl under Ar at the first reduction wave (final
Complex Experimental νCO (cm–1) DFT (cm–1) [Re(pdbpy)(CO)3Cl] (1) 2023, 1918, 1901 2021, 1921, 1902 [Re(bpy)(CO)3Cl] 2021, 1914, 1897a 2017, 1911, 1901 [Re(pdbpy)(CO)3Cl]‒ (1a) 2002, 1891, 1871sh 1999, 1892, 1875 [Re(pdbpy)(CO)3MeCN] (1b) 2017 2017, 1917, 1899 [Re(bpy)(CO)3MeCN] 2011, 1895(br)a 2007, 1898, 1892 [Re(pdbpy)(CO)3MeCN]‒ (1c) 1993 1989, 1890, 1869 [Re(pdbpy‒H)(CO)3] (1d) 2010, 1901, 1879 2008, 1901, 1888 [Re(pdbpy‒H)(CO)3]‒ (1g) 1985, 1865, 1850 1986, 1870, 1861 [Re(pdbpy‒2H)(CO)3]‒ (1h) 2002, 1890 1997, 1888, 1871 a from ref.40
Table 1. Selected experimental (IR-SEC) and calculated νCO frequencies of 1 and other related complexes from
IR spectroelectrochemistry in MeCN and DFT computed data.
Re CO Cl N N Ph OH OH I 1 e-1a +MeCN - Cl -1b HO 1d - 1/2 H2 - MeCN I 1 1 e-1h - 1/2 H2 1g OC CO Re CO Cl N N Ph OH OH I OC CO Re CO N N Ph OH OH I OC CO N Re CO N N Ph OC CO O HO ReI CO N N Ph OC CO O I Re CO N N Ph OC CO O O - Cl -- 1/2 H2
IR-SEC of 1 in anhydrous acetonitrile under Ar at the second reduction wave (Fig. 7) shows that the bands at 2010, 1901 and 1879 cm–1 are replaced by 2002, 1985, 1890, 1865 and 1850 cm–1. The species (1d), formed after the first reduction,undergoes further reduction (Scheme 2) leading to species (1g) (1985, 1868 and 1850 cm–1) which could subsequently generate (1h) (2002 and 1890 cm–1) after loss of ½ H2 (where the charge may
now formally be localized on the oxygen atom).
2050 2000 1950 1900 1850 1800 0.00 0.05 0.10 0.15 A b so rban ce / cm-1 20101985 2002 1901 1879 1865 1850 1890
Figure 7. IR spectroelectrochemistry of 1 in MeCN/0.1 M Bu4NPF6 under Ar at the second reduction wave
(final potential –1.7 V vs. Ag/AgCl).
IR-SEC spectra of 1 in MeCN under CO2-saturated solutions at the first reduction peak are quite similar to
those obtained under Ar atmosphere, with a small exception of the growth of a peak at 1684 cm–1 (Fig. S5), assigned to the presence of HCO3–/CO32– species.40 During the reduction at the second peak under CO2, IR-SEC
at early stage (Fig. S6) rapidly reach a steady-state situation (Fig. 8): the bands at 2010, 1901 and 1875 cm–1, close to those found for (1d) (2010, 1901, 1879 cm–1; Fig. 5, Table 1), remain almost unchanged, while new strong νCO bands at 1684, 1645 (HCO3–/CO32– system40) increase continuously. Under these conditions
stationary concentration of (1g) is too small to be detected: only a weak shoulder of its strong absorption is present at 1985 cm–1 (Figs. S6 and Fig. 8).
2100 2000 1900 1800 1700 1600 1500 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 Absorbance / cm-1 2010 1900 1875 1684 1645
Figure 8. IR spectroelectrochemistry of 1 in MeCN/0.1 M Bu4NPF6 under CO2: steady state established at
continuing electrolysis at the second reduction wave (final potential –1.7V vs Ag/AgCl).
It is interesting to underline that passing from 3 (Mn(pdbpy)) to 1 (Re(pdbpy)), the same ligand has an important effect on the electrochemical properties: the hydride formation in Re is apparently supressed. No formate production has been observed during exhaustive electrolysis (see below).
The active catalytic species are supposed to be the radical anion (1g) and/or the anion (1h), obtained by loss of ½ H2 from (1g). By analogy with the Muckerman and Schaefer mechanism,41 CO production may involve a
carbonate-bridged complex. In that paper the authors highlighted the affinity of Re with O atom, allowing the CO2 insertion into the carbonate-bridged complex. O atom in 1 is close to Re, resulting in the formation of a
Re-O bond of the entropically favoured intermediate (1d), which is reduced to 1g (Scheme 3). Preliminary DFT calculations suggest that an alternative mechanistic pathway, not involving carbonate-bridged intermediate, may also be possible for 1. A CO2 insertion into the Re-O bond of the reduced species 1g (or 1h) is feasible,
and results into carbonate species (Figure S7) that may further react with a second CO2 molecule leading to CO
Scheme 3. Proposed mechanism of the electrochemical reduction of CO2 into CO with 1 as catalyst (similar
to the mechanism proposed in reference 41).
Controlled Potential Electrolysis (CPE)
Bulk electrolysis with 1 and 2 under CO2 were performed upon setting the potential at values slightly
negative to the first and second reductions, with and without added Brønsted acids (water and methanol, 5%). Table 2 summarizes the results obtained during these CPE experiments. A CO2 flow of 30 mL min–1 was kept
constant during the experiments, CO and H2 were identified by gas chromatography, and formate, if present,
was assessed by NMR spectroscopy at the end of the experiments. In MeCN/H2O at the first reduction (–1.5 V
vs. Ag/AgCl, Fig.9a) the TONCO for 1 (7.5) is lower than the TONCO of the corresponding Mn complex 3
(28).31 The fact that TONCO for 2 (E = –1.6 V vs. Ag/AgCl) increases to 11.4 also suggests that 1 is a less active
electrocatalyst. The situation is inverted at the second peak potential, where TONCO of 1 rises to 14.1 (E = –1.7
V vs. Ag/AgCl, Fig.9b), whereas TONCO of 2 drops to a very low value of 2 (E = –1.9 V vs. Ag/AgCl) (Table
4). This is due to the above-mentioned adsorption phenomena onto the GCE that quickly passivate the electrode surface. As in cyclic voltammetry experiments, simple mechanical cleaning of the electrode could restore the catalytic activity. Figure 9 also highlights the good electrocatalytic properties of 1 over time.
Complex E [V] T [min] Acid [5%] TONCO ηCO [%]
1 –1.5 120 270 270 - H2O MeOH 2.2 7.5 4.0 60 76 52 1 –1.7 150 300 240 - H2O MeOH 2.9 14.1 4.4 49 88 64 2 –1.6 270 250 170 - H2O MeOH 6.6 11.4 5.9 98 100 100 2 –1.9 170 100 130 - H2O MeOH 3.7 2 0.7 70 70 60
Table conta a) b) Figur soluti Pho Som comp visibl (Ir(pp cataly from e 2. TON a aining 1 or 2 re 9. Cataly ions of 1 (0 otocatalytic me photo-s plex 2 exten le light irr py)3) was us ysts were ac 89% to 97 and faradic 2 (0.5 mM) ytic CO pro .5 mM) at: c Experime timulated c nsive decom radiation ( sed as a ph ctive toward 7% depend c efficiencie in 0.1M Bu oduction fr a) –1.5 V w ents conversion mposition pr > 420 n otosensitize ds CO2 with ding on the es (η) upon u4NPF6/MeC om CO2 as with 5%v H2 of the CO revented to nm) in acet er and trime h excellent s experimen n CPE (app CN in the pr s a function 2O; b) –1.7 O2 was inve obtain repr tonitrile as ethylamine selectivity f ntal conditi plied potent resence of B n of the cha V with 5%v estigated w roducible da s solvent. f was emplo for CO2 red ons, and a tial E in V Brønsted ac arge during v H2O. with catalyst ata. All exp fac-(tris-(2-yed as a sa uction (Tab maximum V vs. Ag/Ag cids (5%v). g CPE carri ts 1, 3 and periments w -phenylpyri acrificial ele ble 5 and Fi m total TON gCl) of sol ied out in M d 4, where were driven dine))iridiu ectron dono gure 10), ra N of 143 fo lutions MeCN as for under um(III) r. The anging for the
produ exper reduc absen traces concl in the initial as by produ TON M H2 close 1 or 3 3.9 Ir(ppy sensit produ beyon uction of CO riments con ction produc nce of light s of CO (TO lusions hold e absence of l quantity o y-product (2 uce formate of 80 and t 2O, the form to 100 with 3 and the ph 109 M-1 s -y)3* to the tizer (-1.73 uct distribut nd the scope M Mn O and HCO nfirmed tha cts were iss , no produc ON < 3, wh d true in the f the cataly of catalyst, 2.5% in the e with good the Re com mation of C h both 1 an hoto-excited -1 respectiv catalysts. T V vs. SCE tion as com e of this con Mn(pdbpy)(CO n(ptbpy)(CO)3 OO–. All exp at all comp sued from t cts were for hich can be e absence of sts.Turnov similarly to case of 3 d selectivity mpound 1 giv CO2 reductio nd 3. Emissi d Ir(ppy)3 ( vely (see F This result E) is negati mpared to e ntribution, i O)3Br 3Br periments w ponents of the carbon rmed. Unde e explained f sacrificial er numbers o electroche and 3.3% w y and turnov ves 74% se on products ion quenchi (Ir(ppy)3*) w igures S12 falls in lin ive enough lectrochemi it will be stu were repeat the system dioxide. W er argon atm by carbony donor or of s were obtai emical expe with 1). Rem ver. For ex electivity fo s was enhan ing experim with a seco 2-S13), whi ne with the to reduce ical results udied in a fu Re ed two time ms were ne With all the
mosphere, n yl release fr f sensitizer. ined by divi eriments. On markably, b xample, 3 gi r HCOO– a nced (Table ments showe nd order ra ich support fact that s all catalyst may origin uture report (pdbpy)(CO)3C es and gave ecessary for component no products rom the cat
Finally, no iding the m nly small am both Mn an ives 62% s nd a TON o 3), with a ed efficient te constant ts an oxida standard red ts to their a nate from a t. Cl e reproducib r catalytic ts of the sy s were form talyst) and H o products a mol number mounts of H nd Re cataly selectivity f of 86. In th maximum quenching of kq 5.5 ative electro dox potenti active state a mechanist ble results. activity an ystems but med either e H2, and the at all were fo of product b H2 were ob ysts were a for HCOO– he presence TON for fo reaction be 109 M-1 s on transfer ial of the e s. The chan tic change; Blank nd the in the except e same formed by the tained able to and a of 0.5 ormate etween s-1 and r from excited nge in being
Figure 10. TON values vs. time during photochemical irradiation experiments for Mn(pdbpy)(CO)3Br (3),
Re(pdbpy)(CO)3Cl (1) and Mn(ptbpy)(CO)3Br (4). Reaction conditions: 5 µM Cat. + 0.2mM Ir(ppy)3 + 0.05M
TEA + MeCN/CO2 + >420 nm light
TONH2(CS) TONCO (CS) TONHCOO– (CS)
Re(pdbpy) (1) 12.5 (8%)4 (3.3%) a 26 (22.3%) 53 (34%)a 86 (74.4%) 90 (58%)a
Mn(pdbpy) (3) 16 (11.2%)3 (2.5%) a 46 (35.5%) 30 (21%)a 97 (67.8%)80 (62.0%) a
Mn(ptbpy) (4) 5 (5.7%) 24 (30.9%) 50 (63.4%)
Table 3. TON from photo-irradiation (24h) of solutions of 1, 3 and 4 (5 μM) in MeCN in the presence of
Ir(ppy)3 (0.2 mM) and TEA (0.05 M). a: in the presence of 0.5 M H2O as an acid co-substrate.
Concluding Remarks
This contribution provides further insights into the role of a local proton source in CO2 reduction catalysts by
investigating the electrochemical and spectroscopic features of the novel rhenium complexes Re(pdbpy)(CO)3Cl, 1, and Re(ptbpy)(CO)3Cl (ptbpy = 4-phenyl-6-(phenyl-3,4,5-triol)-2,2’-bipyridine), 2 under
inert atmosphere as well as under CO2. Comparison between the rhenium complexes and the corresponding Mn
derivatives Mn(pdbpy)(CO)3Br, 3, and Mn(ptbpy)(CO)3Br, 4, has been also considered. Although the
electrochemical behaviour of 1 in anhydrous MeCN under Ar atmosphere resembles that of a common Re(bpy)(CO)3Cl system, DFT calculations suggest that pdbpy in 1– slightly facilitates the release of Cl– from
the complex, when compared to [Re(bpy)(CO)3Cl]– in the same conditions. Another interesting difference of 1
with respect to other bipyridine-based rhenium catalysts is the presence of small catalytic activity already at the first reduction peak in anhydrous MeCN under CO2 atmosphere. We surmise that the catalytic activity after the
first reduction is associated with the lower energy required for the release of Cl–. The addition of H2O and
MeOH enhances the reactivity of 1 with CO2 on the first and particularly on the second reduction peak.
Compound 2 shows similar behavior at first CV cycles, but in a longer time the adsorption phenomena alter the catalysis. The hygroscopic behavior of 2 does not allow a clean comparison.
Spectroelectrochemical (IR and UV/Vis) investigation coupled with DFT calculations allows the identification of reduced species and interpretation of the reduction mechanisms. Controlled Potential Electrolysis of 1 and 2 under CO2 in MeCN/H2O at the first reduction (–1.5 V vs. Ag/AgCl) shows that the
TONCO of 1 (7.5) is lower than the TONCO of 2 (E = –1.6 V vs. Ag/AgCl), which is probably altered by the
water retained in 2. At this potential there are no evidences of adsorption phenomena by 2. Conversely, at the second peak potential, where increased catalytic performances are expected, TONCO of 1 rises to 14.1 (E = –1.7
V vs. Ag/AgCl), whereas TONCO of 2 drops to a very low value of 2 (E = –1.9 V vs. Ag/AgCl) due to
passivation of the electrode surface. These data apparently suggest that the local proton source effects are more pronounced for Mn in comparison with Re complexes.
In reductive electrochemical experiments it is interesting to note that while Mn(pdbpy) gives the hydride with subsequent production of formate, the corresponding Re(pdbpy) undergoes a different reaction pathway and no formate is produced upon catalytic cycles. It is only during photochemical experiments, under different reaction conditions, that significant formate TON values are obtained, thus showing that the coordination of the pdbpy ligand to Re and Mn alter the selectivity of the reaction products.
Experimental Section
NMR spectra were recorded on a JEOL Eclipse 400 spectrometer (1H operating frequency 400 MHz) at 298 K. 1H and 13C chemical shifts are reported relative to TMS (δ=0) and referenced against solvent residual peaks.
IR-ATR spectra were collected on a Fourier transform Equinox 55 (Bruker) spectrophotometer equipped with an ATR device; resolution was set at 2 cm–1 for all spectra. A spectral range of 400–4000 cm–1 was scanned, using KBr as a beam splitter.
The pdbpy and tpbpy ligands were synthesized accordingly to the Kröhnke reaction between an α,β-unsaturated substrate (chalcone) and pyridinium salt carried out in methanol in presence of large excess of ammonium acetate, as already reported by some of us.30, 31 All reagents were purchased from Sigma-Aldrich and used as received. Solvents were freshly distilled and purged with Ar before use.
Synthesis of [Re(pdbpy)(CO)3Cl] (1). [Re(CO)5Cl] (0.500 mmol, 1 equiv) and pdbpy (0.501 mmol, 1.01 eq)
were refluxed for 3 hours in anhydrous toluene (20 mL) while stirring. Alternatively, reaction proceeded in a sealed flask for microwave reactor and evolved at 130 °C for an hour. After cooling of the reaction mixture to
room temperature, petroleum ether was added to precipitate the yellow product, which was then centrifuged, filtered and washed once with cold diethyl ether (yield 75 %).
1H-NMR: [400 MHz, (CD 3)2SO]: δ/ppm 9.60 (s, 1H), 9.56 (s, 1H), 9.05 (t, J = 8.8 Hz, 2H), 9.01 (s, 1H), 8.35 (t, J = 7.4 Hz, 1H), 8.11 (d, J = 7.8 Hz, 2H), 7.85 (s, 1H), 7.73 (t, J = 6.3 Hz, 1H), 7.60 (m, 3H), 7.15 (t, J = 7.5 Hz, 1H: ), 6.45 (t, J = 7.5 Hz, 2H). (Fig. S11a) 13C-NMR: [400 MHz, (CD 3)2SO]: δ/ppm = 198.87, 194.61, 191.23, 160.25, 157.47, 156.99, 156.70, 156.58, 153.00, 150.25, 140.24, 135.61, 131.10, 129.89, 129.46, 128.77, 128.19, 126.73, 125.46, 120.09, 118.03, 107.35, 106.64. IR-ATR: ν/cm–1 = 3244, 2022, 1911, 1885.
Synthesis of [Re(ptbpy)(CO)3Cl] (2). The reaction proceeded similarly to [Re(pdbpy)(CO)3Cl].
([Re(CO)5Cl] (0.500 mmol, 1 equiv) and ptbpy (0.501 mmol, 1.01 eq) were refluxed for 3 hours in anhydrous
toluene (20 mL) while stirring (yields 70%).
1H-NMR: [400 MHz, (CD 3)2SO]: δ/ppm =9.06 (d, J = 7.47 Hz, 2H), 8.96 (s, 1H), 8.35 (t, J = 8.57 Hz, 1H), 8.15-8.18 (m, 2H),7.96 (s, 1H), 7.75 (t, J = 7.03, 1H), 7.59-7.62 (m, 3H), 6.54 (d, J= 27.97 Hz, 2H). (Fig.S11b). 13C-NMR: [400 MHz, (CD 3)2SO]: δ/ppm = 196.92, 194.41,191.76, 181.02, 164.59, 157.43, 156.86, 153.19, 150.47, 146.5, 140.28, 135.66, 135.61, 133.04, 131,29, 129.89, 128.38, 127.90, 125.96,124.86, 120.26, 109.34. IR-ATR: ν/cm–1: 3226, 2026, 1913, 1877.
The experimental setup is similar to that previously reported.31 Electrochemical experiments were performed using a Metrohm Autolab 302n potentiostat; acetonitrile was freshly distilled and purged with argon before use, tetrabutylammonium hexafluorophosphate (Bu4NPF6, Sigma–Aldrich, 98%), employed as
supporting electrolyte, was recrystallized twice from ethanol and dried before use. The reference electrode Ag/AgCl (KCl 3M) was employed. Under these experimental conditions the redox couple Fc+/Fc is located at E1/2= 0.35V (ΔEp= 56mV).
DFT calculations were performed as previously reported,31 employing the B3LYP method coupled with the
basis set def2TZVP for heavy metals and def2SVP for lighter elements. IR-SEC spectroelectrochemistry were performed in a OTTLE cell,42 analogously as previously reported.31
Quantitative analysis of CO2 reduction products. CO was detected and quantified using an Agilent 490
Micro GC gas chromatograph equipped with a CP-Molsieve 5 Å columns, which was kept at 85°C and at a pressure of 21 psi, with a thermal conductivity detector. The carrier gas was He. The backflush vent option time was set to 4.5 s. A second module equipped with a CP-Molsieve 5 Å column using Ar as carrier gas was used for H2 analysis. The gas inside the measurement cell was sampled for 30 s every three minutes to fill the Micro
GC 10 µL sample loop, and eventually 500 nL were injected into the column for the analyses. We used Ar, He, CO2 pure gases (>99.9995%) from Sapio for the operation, and two different certified standard concentrations
of CO and H2 in Ar matrix (Rivoira) for calibration (Fig. S8). Detection limits for CO and H2 were 1 ppmv and
0.5 ppmv, respectively. Formate production was quantitatively evaluated using NMR spectroscopy.
Single-Crystal X-ray diffraction. Data for compound 1 have been collected on a Gemini R Ultra
diffractometer using graphite-monochromated Mo Ka radiation (k = 0.71073 Å) with the ω-scan method. Cell parameters were retrieved using CrysAlisPro43 software, and the same program has been used for performing data reduction, with corrections for Lorenz and polarizing effects. Scaling and absorption corrections were applied by the CrysAlisPro multi-scan technique. The structure was solved by Patterson Function using Sir201444 and refined with full-matrix least-squares on F2 using SHELXL45. All non-hydrogen atoms were anisotropically refined. Hydrogen atoms were calculated and riding on the corresponding atom. Structural illustrations have been drawn with Mercury.46 The crystallographic data for 1 have been deposited within the Cambridge Crystallographic Data Centre as supplementary publications under the CCDC number 1857181. This information can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cifcode CCDC.
Photocatalytic experiments. Irradiation of either CO2- or argon-saturated 3.5 mL solution containing the
catalyst, the photosensitizer and the sacrificial electron donor were conducted in a closed 1 cm × 1 cm quartz suprasil cuvette (Hellma 117.100F-QS) equipped with home-designed headspace glassware for further gaseous product quantification. A Newport LCS-100 solar simulator, equipped with an AM1.5 G standard filter allowing 1 Sun irradiance and combined with a Schott GG420 longpass filter and a 2-cm-long glass (OS) cell filled with deionized water to prevent catalyst absorbance and to cut off infrared and low ultraviolet, was used as the light source and was placed at right angle of the sample. In these experiments, gaseous products analysis
was performed with an Agilent Technology 7820A gas chromatography (GC) system set with a CarboPLOT P7 capillary column (25 m length and 25 mm inner diameter) and a thermal conductivity detector. Calibration curves for H2 and CO were established separately. Formate was quantified with a Dionex ICS-1100 Ionic
Chromatography System equipped with a IonPac AS15 column (KOH 20 mM as eluent). UV-Visible absorption spectra were recorded with an Analytik Jena Specord 600 spectrophotometer. Emission quenching measurements were conducted with a Cary Eclipse fluorescence spectrophotometer (Agilent Technologies), with the excitation wavelength set at 420 nm. Emission intensities used for the Stern-Volmer analysis were taken at 517 nm, i.e. the emission maximum of Ir(ppy)3.
AUTHOR INFORMATION
Corresponding Authors
Roberto Gobetto (roberto.gobetto@unito.it) and Carlo Nervi (carlo.nervi@unito.it), University of Torino, Department of Chemistry, via P. Giuria 7, 10125 Torino, Italy.
ACKNOWLEDGMENT
J.F. thanks the Czech Science Foundation (project 18-09848S) for support. H.R. and B. M. thank the China Scholarship Council for his PhD fellowship (CSC student number 201507040033 (H.R.) and 201707040042 (B.M.)). Partial financial support to M. R. from the Institut Universitaire de France (IUF) is gratefully acknowledged. PHOTORECARB (University of Torino and Compagnia di San Paolo) and CRT (Fondazione CRT, Ref. 2017.0812) projects are acknowledged.
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